Mass spectrometer



Sept. 9, 1958 Q F, RQBlNsON 2,851,608

MASS SPECTROMETEF.

Filed Deo. 7, 1956 TRANS/FORMER /3 IN VEN TOR* CHARLES F. ROBINSON Afrm/v5 rs United States Patent O MASS SPECTROMETER Charles F. Robinson;Pasadena, Calif., assignor to Conu solidated Eiectrodynamics'Corporation, Pasadena, Calfga corporation of CaliforniaApplicationDecember 7, 1956, Serial'No. 626,860 lClaims. (Cl.-250-41.9)

The invention relates to trometers.

There is a type of mass spectrometer, to date lfinding only relativelylimited use, ingwhich lirst order velocity and angular aberrations arereduced essentially to zero.

double-focusing massY spec- Because: thisk type .of instrumentelfectively focusses` with0 This type of instrument is becomingincreasingly im portant in the `analysis of materials of very low vaporpressure, such as metals, in whichions aremade available over a wideenergy range and hence with an appreciable Spread in initial Velocity. Aspark iony source is a=highly useful source in the ionization` of solidsamples, but it does produce ions of widely ditferingenergies. In 1934,Messrs. Mattauchand Herzog published a paper describ-v ing thedouble-focusing instrument and as-a resultthis type of mass spectrometeris frequently referred to as the Mattauch mass spectrometer (seeZeitschrift fr Physik, 89, p. 786, 1934), In spitel ofthe developmentofthebasic precepts of this form ofinass spectrometer-by Mattauch intheearlyV 1930s, mass spectrometry has been used primarily for the analysisof gases and readily vaporizablel liquids', with which constituentsionization can be accomplished by means of an electron beam and'withavery narrow energy spread; IOniZation` in suchan electron beamvsusceptible to `a high degree ofcollimationv results in velocity.dispension that has usually been negligible in the past, anddouble-focusing instrumentsliaveaccordingly found little use. analysisof compoundsof very= low` volatility isa-1 logical extension of thecapabilities of mass spectrometry.

In the Mattauch mass spectrometer ofY conventional type the electric andmagnetic sectors deilect the ions through angles of 31.8 and 90respectively, .andthesysa tem showsrdouble-focusing (angular andvelocity focus-.H

ing) to first order for allmassessimultaneously.

For anyl mass Width w on the angular aberration a Iand the fractionalvelocity aberration can be. expressed as a power series in a and asfollows:

CH is a constant of' fractional velocity aberration as a.

function of the -instrument parameters, and is zero by definition indouble-focusing mass spectrometers..v

In the standard Mattauch instrument, C and Ca are More recently` it hasbecome.` apparent` that: the.

spectrometer the dependence of image,Y`

zeroY at the focal planevby denition, and Ca2, Ca, and` CB2 are given bythe following expressions:

re=radius of curvature of electric sector; rm=radius of curvature of aspecifiedV ion in the magnetic sector; and Y:separation of electric andmagneticsectors.

It ,can be shown by plotting the functions Ca2, Ca and.l standardMattauch system to. and magnetic sectors (d) as'small as mechanicalconsiderations will permit in.

Cf thatit-,is desirable in a keep the separation of the electric orderto keep Cali and C),2 from becoming excessively large.' If this is doneit becomes ditlicult to effect clean and independent control of a andThis fact together with the fact that C,ZB and reasonable intensitysimultaneously when the velocity spread in the ion beam source or `anionbombardment ion source.

that by curving the entrant pole face of the magnetic sector, convexlywith respect to the entering ion beam andon av radius of' curvature p,leaving theelectric and,` magnetic deflections at their normal values of31.8 and` 90 respectively, the second order effects of angular andvelocity aberration can bel reduced to a minimum. As

a result ,thereV is realized greatly improved resolving: powerv comparedto a` standard double-focusing instrument.

Accordingly, the invention comprises a mass spectrometer having an ionsourceanV electrostatic velocity com- 4 pensator, means including spacedmagnetic pole pieces forming a magnetic eld, the entrance faces of themagnetic pole pieces being convex, means for causing ions formed at thesource to pass sequentially through the electrostatic velocitycompensator and the magnetic eld, and meansffor sensing ions passingthrough the magnetic field.v

The analysis of the consequences of this alteration in instrumentstructure is deferred detailed description of a double-focusing massspectrometer in accordance with the invention as 4taken fr'orntheaccompanying schematic drawing. V

Referring -to the drawing, the instrument shown schematically includesan ion source, in this instancet illustrated as spark electrodes 10 and11, connected at opposite ends to the secondary winding 12 of a Teslatransformer-#13. The lmidpoint of thetransform'er'winding-is connectedtol thefpositive4 sideof a high voltage supply 14;

First,l second and third apertured electrodes 16," 17

and 18fare; arrangedfserially, electrode 16 nected'through a suitable`voltage divider 20 to the high voltage supplyzand` serving `as anlaccelerating'electrode to:v propel ions `frta'n the spark'source'as-afbeam through" thefelectrodes 17 and 18. Electrode. 17 has anaperture' definingthe4 geometrical-width of the beam, andV S eloectrode18 has an aperture S1; acting. as a .limit4 on angular divergence-,inthebeam passing.I therethrough;L

An electrostatic deector constituting curved plates 22, 23 is arrangedserially with respect to the electrodes, the beam issuing from theelectrode 'array and entering the deector, as shown. The two plates, 22,23 of the deilector Ca2 are not very small mayV make it very ditiicultto achieve high resolving power and is high as from a spark ion Inaccordance with the present invention I have found.

pending the following"vv are connected to opposite sides of a deectingvoltage supply 24 and, as illustrated, both plates are at a preassignedpotential with respect to ground 25. A fourth apertured electrode 26 maybe disposed adjacent the exit end of the electrostatic deector.Electrode 26 has an aperture S2 acting as a so-called velocity stop,i..e. stopping ions which, because of extremes in velocity dispersion,are not focused on the aperture S2.

A transverse magnetic field is formed in the conventional manner by apair of magnetic poles, one of which, pole piece 28, is shown in thedrawing. In accordance with normal practice, the second identical polepiece (not shown) is disposed parallel to and spaced from the pole piece28 to form a magnetic eld therebetween transversely of the direction ofion travel into the magnetic eld from the deector. Ion sensing means,such as the photographic plate 30, is immersed in the magnetic eld fordetecting mass dispersion therein.

' Unlike previous Mattauch type mass spectrometers, the pole piece 28and its companion pole piece (not shown) which together detine themagnetic eld, are provided with curved entrant pole faces, pole piece 28having curved face member 29 which is convex with respect to theentering ion beam and resulting in the improved operation as pointed outbelow. Preferably the convex face is provided as a detachable shim, asshown, held to the parent face surface by any desired means, such asbolts 31, 32. In this manner, the radius of curvature (p) can b'echanged at will by the simple expedient of changing shims.

Certain parameters are identied in the drawing to facilitate thefollowing discussion of the operation of the instrument. Thus thenotation re gives the mean radius of curvature of the electrostaticdeector, the notation d shows the distance between the exit from thedeector to the entrant face of the magnetic field, the notation rm (max)shows the maximum radius of ion travel in the magnetic eld, and thenotation p gives the radius of curvature of the entrant face of the polepieces.

With a curved entrant pole face the second order factors from Equation 1can be shown by mathematical derivation not included herein to be asfollows:

r=the mean radius of curvature in the electric sector; d=sectorseparation; and

In a conventional double-focusing instrument the pole face of the magnetis not curved in effect, p is therefore infinite, 'y equals one and thesecond order abberation coefticients as given in Equations 2 are ofappreciable significance. Where p is less than innity in accordance withthe present invention, y is less than one and, by proper choice ofinstrument parameters, i. e. of re, rm, d and p, the second order ofEquations 3 can be reduced to an insignificant minimum. It can be shown,although no attempt will be made to do so here, that the third orderaberrations are negligible.

Itis not possible to find positive values for and rm such that all threeof these coefcients vanish from the above equations. For this reason itis not possible to achieve values in which Cu2, Cmq and Cf all becomezero at the same time. However, it is possible to nd a considerablerange of values for these parameters such that CM and C coefficientsvanish simultaneously and Cu2 is of quite reasonable magnitude. Equallyimportant is the fact that this condition can be met for values of dwhich are not as small as those heretofore thought to be necessary butroughly equal to r,I1 so that the operation of the energy selector slit(26 in the drawing) is greatly improved over its function in the moreusual form of the Mattauch instrument. Not only is the operation of theenergy slit improved, but its operaiton is no longer as critical sinceCas and C2 have been reduced to zero.

As an aid to the selection of values of rm and y, it can be shown bymathematical derivation that for an arbitrary value of and with theobjective of reducing Cas and C=0, the

ratio may be selected in accordance with the following expression:

and'y may be determined from:

Second order aberration coefficients for a Mattauch mass spectrometerhaving straight pole faces in which re=20.2, d=2.4

r- Caz Cup C TABLE 2 Second order aberration coefficients for a Mattauchmass spectrometer in which the leading face of the magnet pole has aradius of curvature p=4.265, re=24.876",

r. C s C ap C ai The Vchoice of parameters von `which Table '2i's"b'a'se'd is not intended to evidence an optimum `'set of valuesbutone which might be reasonable for mechanical convenience. It is noted,however, that both energy dependent aberrations vanish at the designpoint r,=12" and that the arrangement reported in Table `2l gives alower aberration 4sum (C2{Ca,}-C,2) throughtheentire'mass range thandoes the conventional instrument evidenced in Table 1.

Referringto the'drawing, typical'values for structural and operationalfeatures of the instrument may be as follows:

r1n. may be selected in the design of an instrument such that for aparticular value of Iy, as for example determined from Equation 5,Ca=C2=0- Where Ca and Cz: under such circumstances Ca2 does not equalzero. With the same values of and i; a different value of v can be foundsuch that Ca2=0 and in which case Cag and C32 do not equal zero. Withthe objective of diminishing Ca2 to 0,

and

I'1n may again be determined from Equation 4 and 'y as follows:

rm 2 Mr.)

Table 2 shows a selection of parameters such that Cn, and C52 are 0 andunder which conditions Ca2 is not zero.

With this option to select 'y (by variation of p) so as to eliminateeither Cm, and CH2 or Ca2 it is possible without changing the fixedgeometry of the instrument (re, rm and d) to specifically adapt it touse with different types of ion sources. Where a source is used whichproduces an extreme energy spread such as the illustrated spark source,the principal objective is to reduce CM and CB2 to a minimum withoutundue disturbance of Ca2. However, if a surface ionization source isused, the energy spread is so low that the a and ,92 aberrations do notwarrant concern at the expense of the a2 aberration. In such event,thereof, a value of 'y is selected to reduce Ca2 to zero with little orno concern for Cals and C52. By using the same geometry specified inTable 2 save for a change in p from 4.265 to 35.5, Ca2 is reduced tozero and the Cap and CB2 coeiiicients become 7.258 and 2.460respectively. From Equation l it is apparent that the value of theselast two coefficients fi is of no signic'ance where 5e surfaceionization `r source.

Therefore, without altering the instrument geometry, I amable to securemaximum resolving power for any given conditionsrofionizationbytchangingthelradius of curvature of the entrant faces of themagnet poles. This is simply accomplished by the use of removable polefaces as illustrated.

`I claim:

"1. A mass spectrometer lcomprising an ion source, an electrostaticcompensator, means including spaced magnetic pole pieces forming amagnetic field, means'for causing -ions 1formed `in the ysource to`travel .serially through the electrostatic compensator and the magneticfield, the entrant faces of the magnetic pole pieces being convex in thedirection of ion entry, and means for sensing ions passing through themagnetic field.

2. A mass spectrometer comprising an ion source, au electrostaticcompensator, means including spaced magnetic pole pieces forming amagnetic field, means for causing ions formed in the source to travelserially through the electrostatic compensator and the magnetic field,an apertured electrode disposed between the compensator and the magneticfield, the entrant faces of the magnetic pole pieces being convex in thedirection of ion entry, and means for sensing ions passing through themagnetic field.

3. A mass spectrometer comprising an ion source, an electrostaticcompensator, means including spaced magnetic pole pieces forming amagnetic field, means for causing ions formed in the source to travelserially through the electrostatic compensator and the magnetic field,face plates removably attached to the entrant faces of the magnetic polepieces, the outer surface of the face plates being convex in thedirection of ion entry, and means for sensing ions passing through themagnetic field.

4. A mass spectrometer comprising a spark source of ions, anelectrostatic compensator, means including spaced magnetic pole piecesforming a magnetic field, means for causing ions formed at the sparksource to travel serially through the electrostatic compensator and themagnetic field, face plates removably attached to the entrant faces ofthe magnetic pole pieces, the outer surface of the face plates beingconvex in the direction of ion entry, and means for sensing ions passingthrough the magnetic field.

5. A mass spectrometer comprising an ion source, an electrostaticcompensator, means including spaced magnetic pole pieces forming amagnetic field, the entrant faces of the pole pieces being convex, meanscausing ions to flow from the source through the compensator andmagnetic field, means for sensing ions passing through the magneticfield, the dimensions of the instrument being determined from approacheslzero nas in Va and 32+ m/rm 7:7mm/mw d/f...) (f1/f...) where:

rm=the radius of curvature in the magnetic sector; r=the mean radius ofcurvature in the electric sector; d=sector separation;

ln=radius of curvature of entrant face of magnetic pole pieces.

and

where:

r,=the radius of curvature in the magneticfsector;

re=the mean radius of curvature in the electric sector; d=sectorseparation;

and p=radius of curvature of entrant face of magnetic pole pieces.

References Cited in the file of this patent UNITED STATES PATENTSWashburn Dec. 10, 1946 Oppenheimer et al Oct. 4, 1955 Patent No.2,851,608

UNITED STATES PATENT OFFICE Certificate of Correction September 9, 1958Charles F. Robinson It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionand that the said Letters Patent should read as corrected below.

Column 4, line 87, for Ue/01,0, which is the last quantity of thedenominator' of the equation, read -(r/m) and this equation should bedesignated as -(5)-; column 4, line 59, Table 1, last Column thereof,for 1.3 read -1.32-; same column, line 74, Table 2, last column thereof,for 10.92 read -10.9-; Column 5, lines 53 and 54, the equation n Tm 2n1:2 8 E) should be designated as -(6)-; column 6, lines 65 to 68, claim5, for

'yd-127272 read Fy=1--"; and

Signed and sealed this 2nd day of December 1958.

[eEAL] Attest:

KARL H. AXLINE, ROBERT C. WATSON, Attest'ng Ojoer. ommz'ssz'oner ofPatent/8.-`

